Micro-devices, or micro-machines, as discussed herein, are devices, often mechanical, electrical, or both, in nature, less than 200 microns in size. Structures and devices of this size and smaller can be built in many ways, such as using MEMS (micro-electro mechanical systems) techniques, which often employs lithography to create microscopic structures (and thereby overlap with integrated circuit manufacture techniques), self-assembly, micro-machining, or any other suitable technique. Note that while an entire micro-device may be up to 200 microns in size, its individual parts may be much smaller. For example, the state of the art in integrated circuit lithography allows features of 22 nm or smaller, self-assembly allows the creation of structures based on individual molecules, and AFM-based technologies allow the placement individual atoms. The methods of construction of micro-devices are numerous, and known to those skilled in the art.
Micro-devices have many potential applications. For example, in the medical field implanted micro-devices could provide high-resolution, real-time measures of many properties. Important solutes could be measured (e.g., glucose, sodium, potassium, calcium, bicarbonate, etc.), as could physical properties such as temperature and pressure. Current examples of macro-scale devices directed at performing similar functions include pill-sized cameras to view the digestive tract as well as implanted glucose and bone growth monitors to aid treatment of diabetes and joint replacements, respectively. The development of micro-devices significantly extends the capabilities of such machines. For example, clinical magnetic resonance imaging (MRI) can move micromachines containing ferromagnetic particles through blood vessels. (Ishiyama, Sendoh et al. 2002; Martel, Mathieu et al. 2007; Olamaei, Cheriet et al. 2010)
Other demonstrated micromachines use flagellar motors to move through fluids, and offer the possibility of minimally invasive microsurgeries in parts of the body beyond the reach of existing catheter technology. (Behkam and Sitti 2007; Fernandes and Gracias 10, 2009). The uses for such devices are numerous and extend beyond the field of medicine to uses such as basic research and industrial applications. Note that while exemplary uses are described herein, others will be apparent to those skilled in the art. It should be recognized that the value in the present invention resides in the general principles provided for powering and communicating with micro-devices of many different types, in many different environments, not just those mentioned or shown in the embodiments.
Providing power to the micro-devices is a challenge. For example, power from batteries would be limited by their small size and power harvested from the environment is limited by available energy sources and the complexity of manufacturing power generating components at small sizes. Other techniques, such as inductive powering and other forms of wireless power transmission can be hampered by the frequencies needed to efficiently couple to the micro-device and by attenuation in tissue.
Communication poses a challenge for micro-devices. Small overall device size limits antenna size, which makes selection of wavelengths which can be adequately coupled to a micro receiver or transmitter problematic. Further, the optimal modes of communication of a micro-device with a macro-scale transceiver may differ from the optimal modes of communication between micro-devices. Communication between micro-devices can address several problems. For example, such communications could enable machines to coordinate their activities, thereby providing a wider range of capabilities than having each machine act independently of others. For instance, nearby machines could compare their measurements to improve accuracy by averaging noise, determine gradients or identify anomalous behaviors such as the failure of one machine. Such communication could also allow the machines to combine their measurements into compressed summaries, thereby reducing the amount of information necessary to communicate to the external receiver. And, communication between micro-machines enables data to be sent to the micro-machine closest to the external receiver, thereby improving the transfer of information to the receiver.
The small size of the micro-devices is not the only challenge to providing power and communication. Micro-machines may operate within environments which raise additional challenges. For example, in the body, tissues, including blood, different organ tissues, and bone, may have physical properties that are not well-characterized at the small sizes relevant to micro-machines and such properties can vary over millimeter distances. The tissue properties may affect transceiver and micro-machine design and performance due, for example, to its attenuation characteristics.
Acoustics are one approach to coupling power and data transmission to micro-devices. Sound is readily transmitted through many materials and is easily produced by machines. Ultrasound has been used to communicate with conventional, large-scale implants, and micro-devices can use piezoelectric materials to produce sound. However, the small size of micromachines makes them inefficient at converting vibration into sound waves at the frequencies commonly used for larger devices. Micro-devices are more efficient at generating higher frequency sounds. However, some environments (for example, biological tissue) significantly attenuate high-frequency sound. Compensating for inefficiency or attenuation would require significant power, which may not be available to the devices. And, in biological settings, even if sufficient power were available, increased power could lead to localized tissue damage due to intense power flux at the surface of the machines. Overcoming these problems requires creating a sound field adapted for transmission through various environments such as water or other fluids, blood, tissue, industrial chemicals or waste, or other environments, through suitable choices of operating frequencies, surface motions and calibration. This procedure requires different choices for sending sound from the micro-machines to each other, from a micro-machine to the external receiver and from the external transducer to the micro-device.
Acoustics, in the form of ultrasound, has been used for imaging, cleaning and agitation, industrial and biological measurement and testing, the enhancement of drug delivery (U.S. Pat. No. 7,985,184, 2011) as an adjunct to antibiotic therapy (and other uses related to cell permeability), for welding, for USID (ultrasound identification), and more. Micro-devices capable of generating ultrasound have the potential to provide similar functions, if the attendant problems with small device size can be overcome.
(U.S. Pat. No. 7,570,998, 2009) “Acoustic communication transducer in implantable medical device header,” teaches an implantable medical device containing an ultrasonic transducer. Communication between the device and an implanted sensor occurs using frequencies in the 10-100 kHz range. These frequencies are suitable for conventional devices, but not micro-scale devices.
(U.S. Pat. No. 7,945,064, 2011) “Intrabody communication with ultrasound,” teaches the use acoustics as an alternative to RF transmission. This describes macro-scale ultrasonic transducers using part of body as a communication channel at frequencies between 100 kHz and 10 MHz. This does not teach the use of micro-devices, and the frequencies are generally too low to efficiently couple to micro-devices.
(U.S. Pat. No. 8,040,020, 2011) “Encapsulated active transducer and method of fabricating the same” teaches MEMS-based ultrasound generators. Specific applications (e.g., communication within tissue) are not discussed.
(U.S. Pat. No. 8,088,067, 2012) “Tissue aberration corrections in ultrasound therapy,” teaches adjusting ultrasound for tissue inhomogeneities at larger scales for improved focus. The size scales, and attendant challenges, are quite different than the present invention.
Theoretical studies of communication with and among sub-millimeter implanted devices, have been published ((Freitas 1999; Hogg and Freitas 2012), by the inventors and upon which this application is based and which is herein incorporated by reference). However, these studies do not address all of the challenges or details involved in micro-scale communication and power.
While some differences between the invention and the prior art a listed above, a more general observation should be made: The prior art is not directed to surmounting the problems inherent in transceiving sound at small scales and distances.
Specifically, the prior art does not address one or more of the following problems: limitations in available power, power coupling to micro-devices, acoustic attenuation in various milieus (e.g., tissue), efficient acoustic wave generation by micro-devices, thermal noise and its effect on communication rates, safety (in biological settings) or the choice of frequencies based upon communication channel spacing and background noise.